Double strand breaks are very dangerous lesions in DNA and must be repaired to allow cells to complete replication and transcription. Cells with deficiencies in double strand break repair are subject to genomic instability and individuals with these deficiencies are often at elevated risk for cancer. The presence of double strand breaks can be determined by examining cells for the phosphorylated form of a histone protein variant known as H2AX. Phosphorylation of the protein occurs rapidly in the vicinity of a double strand break, and persists until the break is repaired. Consequently phospho-H2AX (aka. -H2AX) serves as a surrogate marker for breaks. This species can be easily detected by immunofluorescence techniques, and thus can be detected in single cells. We sought to test the hypothesis that double strand breaks are present in elevated levels in cells from aged individuals, as compared with younger individuals. At this time, we have completed the examination of -H2AX foci in fresh peripheral blood lymphocytes derived through apheresis from 40 individuals between the ages of 35 and 83. We are currently investigating the correlations between -H2AX foci and different clinical diseases which are known to be associated with genome instability and/or oxidative stress. Cancer, vitamin D deficiency, cataract formation and hypertension, for example, will be examined.